1. Field of the Invention
The present invention relates in general to well monitoring and, in particular, to a well completion method to allow dual reservoir saturation and pressure monitoring.
2. Brief Description of Related Art
In conventional oil and gas production operations, a production well may be drilled into a subsurface fluid reservoir and completed for the production of reservoir fluid to the surface. Generally, a monitoring well may be drilled into the same reservoir as the production well. These monitoring wells provide information regarding the reservoir so that production may be controlled to maintain production at desired levels. The monitored information may include reservoir pressure, reservoir saturation levels, and the like.
The monitoring well will be drilled to the reservoir depth and completed by suspending a casing string within the wellbore. The annulus between the casing string and the wellbore will be cemented to secure the casing string within the wellbore. The casing string may be perforated at the reservoir location to allow fluid flow from the reservoir into the casing string. A tubing string will then be run and set to the casing string through the use of tubing packers so that an end of the tubing string is above the reservoir. A pressure monitoring gauge will be mounted to the end of the tubing string to monitor the reservoir pressure. Saturation and production logging may be performed through the perforated portions of the casing string located at the reservoir. Generally, pressure monitoring, saturation logging, and production logging occur proximate to, or below an end of the tubing string. In particular, saturation logging involves measurement of the pore volume of a reservoir formation that is filled by water, oil, and/or gas. Typically, acoustic or electromagnetic signals are passed into the formation through the monitoring well to generate saturation data. The signals must pass through the casing string wall and the cement layer to penetrate the formation.
Some production wells will be drilled through two reservoirs. In the corresponding production operations, reservoir fluid may be produced from both reservoirs. As a consequence, both reservoirs must be monitored. To obtain accurate pressure and saturation measurements, separate monitoring wells must be drilled to each reservoir. This is extremely costly and inefficient, essentially doubling the cost of reservoir monitoring.
Some attempts have been made to monitor two reservoirs from a single well. In these monitoring well completions, a single wellbore is drilled through both reservoirs. A casing string is then set and cemented in the wellbore. The cement layer in the annulus between the casing string and the wellbore will extend from the bottom of the well to the surface. The casing string is then perforated at both reservoirs. A tubing string is then run and set within the casing string. The tubing string will be set with a lower packer in between the reservoirs and an upper packer above the upper reservoir. Again, pressure monitoring, saturation logging, and production logging for the lower reservoir will all be conducted proximate to or below the end of the tubing string. However, this only provides accurate measurements of the lower reservoir.
Attempts have been made to conduct saturation logging operations across the upper reservoir. When this is attempted, the saturation logging signal must pass through the tubing string wall, an annulus between the tubing string and the casing string, the casing string wall, and the cement layer before entering the reservoir. The addition of the tubing string wall and annulus between the tubing string and the casing string may significantly decrease the strength of the saturation logging signals. As a consequence, the information generated during the saturation logging operations for the upper reservoir may be highly inaccurate. In addition, pressure monitoring is not possible due to the inability to isolate flow from the two reservoirs when a pressure gauge is used to monitor the upper reservoir. Therefore, there is a need for a well completion method that allows for accurate monitoring of pressure and saturation of more than one reservoir from the same well.